N-Acetyl-.beta.-D-glucosaminidase (abbreviated to as NAGase) is an enzyme in lysosome which is contained abundantly in the renal tubular epithelia and is related to the degradation of mucopolysaccharides and glycoproteins. It is recognized that the NAGase in urine increases in various cases of renal diseases or after an operation of the kidney and the NAGase in urine as well as serum increases in case of diabetes. In the field of clinical and animal experiments, the measurement of the NAGase has attracted an interest in connection with diagnosis of various renal diseases and also as an indicator in examining nephrotoxicity.
In the prior art, p-nitrophenyl N-acetyl-.beta.-D-glucosaminide [Biochemical Preparations, 10, 118 (1963)] and 4-methylumbelliferyl-N-acetyl-.beta.-D-glucosaminide [Clinica Chimica Acta. 69(1), 85-91 (1976)] have generally been used in the determination of the NAGase activity. The use of the former, however, has a disadvantage that biocomponents such as billirubin and hemolytic hemoglobin interfere with the measurement at the wavelength employed to require the blank measurement for each sample and so complicate the measurement procedure. The use of the latter has also a disadvantage that special apparatus such as a spectrophotometer are needed. The present inventors have completed this invention as the result of intensive investigations to find substrates for determining the NAGase activity with which the measurement can be performed in high sensitivity, precisely and quickly without the disadvantages mentioned above. It has been described in Jap. Pat. Unexam. Pub. No. 51-114990, etc., that phenolphthaleinyl N-acetyl-.beta.-D-glucosaminide structurally analogous to the compounds of the present invention can be used in determining the NAGase in saliva of pregnant women. This reagent, however, has disadvantages that it is insoluble in the assay buffer and even after solubilization with a solubilizing agent, the assay reaction yields unstable color which is liable to fade soon. The compounds of the present invention have no such disadvantages.